There are two ways to measure this using available technology:
a) fine resolution at low-flow breathing, steady state (under resting conditions)
b) course resolution at sub-maximum exercise conditions, also steady state in incremental stages within exercise (after achieving a steady state first).

Breezing Tracker is designed exclusively for the first method mentioned above: under resting conditions. Breezing’s design meets the purpose of determining one’s Resting Metabolic Rate (RMR) or Resting Energy Expenditure (REE), which is used to calculate the majority of Daily Caloric Need  for those in the Sedentary population:

Daily Caloric Need = Resting Energy Expenditure (REE) + Active Energy Expenditure (AEE)

Technology used to estimate AEE lags  far behind the technology that measures REE. Unfortunately,  accelerometer-based activity trackers, HR (heart rate) assisted or not, are all generating numbers for “Calories Burned” with one-size-fits-all equations that haven’t been validated for accuracy. It is best to adopt a conservative “ball-park” figure for AEE.

Exceptions:
Professional athletes, for whom a large percentage of the day is spent utilizing calories to grow (either mass or caloric storage for performance/endurance), have access to state-of-the-art assessments of Caloric Consumption during exercise. Currently, the instruments are costly (~30,000+ USD) and the assessments are limited in availability due to the difficulty of conducting measurements.